1. Field of the Invention
The present invention relates to a chemical vapor deposition (CVD) process for plasma depositing a silicon carbide film onto a substrate.
2. Brief Description of the Prior Art
Chemical vapor deposition (CVD) is used through the microelectronics industry for semiconducting and insulating thin film deposition. When films such as silicon carbide are deposited, silane (SiH.sub.4) is generally used as the gaseous silicon source.
Silicon carbide (SiC) films are used in solar cells, as X-ray lithographic masks, and as hard coatings on machining tools. Silicon carbide has the necessary electrical properties for use in solar cells and has good physical properties.
Silane is highly toxic and spontaneously ignites when exposed to air. It requires the use of expensive gas cabinets and a cross-purging gas supply system. Special purging procedures are needed before introduction into deposition equipment. A number of silicon containing chemicals have been used or proposed as alternative silane sources. These include silicon tetrachloride (SiCl.sub.4), silicon tetrabromide (SiBr.sub.4), silicon tetrafluoride (SiF.sub.4), dichlorosilane (SiH.sub.2 Cl.sub.2), and disilane (Si.sub.2 H.sub.6) All halogen containing silanes are toxic and corrosive themselves in addition to producing toxic and corrosive by-products. Disilane is a flammable, toxic gas that requires similar handling procedures to silane. Also, use of these alternative silicon sources requires an additional reactant such as methane to supply carbon to the deposition Process to form the silicon carbide film. Other chemicals such as tetraethylorthosilicate [TEOS, Si(OC.sub.2 H.sub.5).sub.4 ] and tetramethylcyclotetrasiloxane (TMCTS, C.sub.4 H.sub.16 Si.sub.4 O.sub.2) can only be used for oxide deposition.
Accordingly, there is a need for better alternatives to silane as a precursor for depositing silcon carbide films in CVD processes.
U.S. Pat. No. 4,459,338, which issued to Angelini, et al. on July 10, 1984, teaches a CVD process for applying silicon carbide onto a film by contacting the substrate with a gaseous mixture of methylsilane and an inert gas while heating the gaseous mixture and the substrate to about 800.degree.-1,050.degree. C.
U.S. Pat. No. 4,810,526, which issued to Ito, et al., on Mar. 7, 1989, teaches a CVD process for applying silicon carbide onto a substrate in a heated furnace by introducing a feedstock gas containing a single silicon and carbon source (e.g., trichloromethylsilane) into the heated furnace under a vacuum.
U.S. Pat. No. 4,923,716, which issued to Brown, et al., on May 8, 1990, teaches a CVD process for depositing silicon carbide from a vapor source having a single molecular species that has both silicon and carbon atoms in equal numbers. Preferred molecular sources include H.sub.3 SiCH.sub.2 CH.sub.2 SiH.sub.3 ; a silacycloalkane of the form (-SiH.sub.2 CH.sub.2 -)p, where p is 2, 3, 4, or 5, or a cyclic structure of the form [-SiH(CH.sub.3)-].sub.q, where q is 4 or 5.
Separately, di-tert-butylsilane (DTBS) is a known chemical with a Chemical Abstracts registry number [30736-07-3]. Processes for making DTBS are disclosed by Wataname, et al. "A Simple and Convenient Method for Preparing Di-t-Butyl Silanes", Chemistry Letters, pp. 1321-1322, 1981; Doyle, et al. "Hindered Organosilicon Compounds, Synthesis and Properties of Di-tert-butyl-, Di-tert-butylmethyl-, and Tri-tert-butylsilanes", J. Am. Chem. Soc. 97, pp. 3777-3782 (1975); and Triplett, et al. "Synthesis and Reactivity of Some t-Butyl-Disilanes and Digermanes", Journal of Organometallic Chemistry, Vol. 107, pp. 23-32 (1976). All three of these articles are incorporated herein by reference in their entireties. DTBS has been used as a silylation agent to hydroxy compounds (CA 101:91218v) and as an intermediate in the production of di-tert-butyldichlorosilane (CA 98:126375t).
Also, DTBS has been used as a silicon source for depositing silicon nitride or silicon dioxide by either thermal or plasma CVD processes. See U.S. Pat. Nos. 4,877,641 and 4,877,651, both of which issued to Thomas S. Dory on Oct. 31, 1989. Both of these U.S. patents are incorporated herein by reference in their entireties.
Di-tert-butylsilane (DTBS) is an air-stable, noncorrosive liquid. It is soluble in many organic solvents and does not react with water. Its high vapor pressure at room temperature allows for easy introduction into CVD reactors. No gas cabinets or cross-purging systems are needed in order to use this chemical in CVD reactors. The decomposition by-products are not corrosive in nature. Also, in the deposition of silicon carbide, DTBS can be used by itself to form the film since it contains the required carbon to form silicon carbide. Di-tert-butylsilane represents an air-stable liquid source that can be used for the deposition of silicon. This eliminates the need for using the flammable, toxic chemical, silane, and eliminates the need for an additional reactant to supply carbon for the deposition process.